Method for the manufacture of a urea-based particulate material containing elemental sulphur

ABSTRACT

This invention relates to a method for the manufacture of a homogeneous, solid, particulate, urea-based material comprising elemental sulphur. The invention further relates to a homogeneous, solid, particulate urea-based material comprising small elemental sulphur phases in a urea-based base material and formed by an accretion process. The product is in particular suitable as a fertilizer.

SUMMARY OF THE INVENTION

This invention relates to a method for the manufacture of a homogeneous,solid, particulate, urea-based material comprising elemental sulphur.The invention further relates to a homogeneous, solid, particulateurea-based material comprising small elemental sulphur phases in aurea-based base material and formed by an accretion process. The productis in particular suitable as a fertilizer.

BACKGROUND OF THE INVENTION

Sulphur-containing fertilizers are in ever larger demand forcompensating sulphur deficiencies in the soil. Conventionally, sulphurhas been applied to the soil in the form of elemental sulphur, or ascompounds such as ammonium sulphate, ammonium bisulphate, thiosulphates,sulphides or gypsum, or in combination with other fertilizer materialssuch as urea, for example as a sulphur-coated urea, as disclosed in U.S.Pat. No. 3,903,333 (Tennessee Valley Authority, 1975) and U.S. Pat. No.5,599,374 (RLC Technologies LLC., 1997).

Sulphur-containing fertilizers, including fertilizers that containelemental sulphur, are already known for a long time, the first patentson fertilizers containing elemental sulphur being issued more than 50years ago. A sulphur-containing fertilizer solves the need to providesulphur as a nutrient to plants. An agronomical benefit for usingelemental sulphur is that a fertilizer comprising elemental sulphur canoffer a higher nitrogen content in the fertilizer in the presence of ahigh sulphur concentration, e.g. over 42 weight % of nitrogen (N) over 8weight % of sulphur (S) in a urea/sulphur fertilizer.

However, in the case of elemental sulphur, as such, it is notbiologically available and needs to be converted to sulphates bybacteria in the soil and dissolved into water, available in the soil, inorder to be of nutritional value to the plant. Therefore, othersolutions have been found, such as the provision of urea-ammoniumsulphate (UAS), in which the sulphur source is dissolvable into waterand does not need a biological conversion.

Lately, new efforts have been devoted to the manufacture of urea-basedfertilizers containing elemental sulphur, manufactured from amelt-mixture of urea-based base material and elemental sulphur.

U.S. Pat. No. 3,100,698 (Shell, 1963) discloses a fertilizer compositionconsisting essentially of co-melted and prilled urea and elementalsulphur. It is manufactured by mixing a liquid flow of fertilizer at atemperature of 141° C. and a liquid flow of elemental sulphur at atemperature of 127 to 142° C. using a pump, and it is prilled using afan spray atomizing nozzle in a classical prilling tower. Vigorousstirring is necessary to avoid phase separation. Instead of employing aprilling tower, the product may be made by other techniques such as bygranulating, spherodizing or flaking. The main disadvantage of thesulphur-urea product made according to this method is that the elementalsulphur does not oxidize rapidly enough to provide nutrient sulphur thatis available early in the growing season and the sulphur becomingavailable only in the later stages of plant growth.

U.S. Pat. No. 4,330,319 (Cominco Ltd, 1982) discloses a process formaking a urea-based fertilizer comprising elemental sulphur by mixingmolten urea and molten elemental sulphur to obtain a molten mixture andsolidifying the molten mixture to obtain a particulate urea-basedfertilizer comprising elemental sulphur, passing the molten urea and themolten elemental sulphur through a mixing device (static mixer) at atemperature above the melting points of the urea and elemental sulphurin relative amounts, sufficient to produce said urea-based fertilizercomprising elemental sulphur, maintaining a pressure drop across saidmixing device of at least about 200 kPa to form a homogenized melt ofurea and elemental sulphur, and solidifying said homogenized melt in aninclined rotating granulation drum to obtain a homogeneous, solid,particulate urea-based fertilizer comprising elemental sulphur whereinthe elemental sulphur phases have a size of smaller than about 100 μm.Essential in this process is the provision of a homogeneous melt by theuse of a mixing tube with a “T” shape for joining the molten elementalsulphur flow with the molten urea flow, a melt that is subsequentlyhomogenized in a mixer, then solidified into solid particles by arotating drum. It is further disclosed that any one of a number of othermethods can also be used, including prilling using a cooling gas in atower, an inclined rotating pan or a fluidized bed.

It was found that a small elemental sulphur phase size was favourablefor an efficient bacterial conversion into sulphates and that the phasesize should be equal to or smaller than 100 μm, preferably equal to orsmaller than 20 μm, for the bacterial conversion into sulphates to befast. Hence, research has been performed to minimize the size of theelemental sulphur phases within the urea fertilizer particles by addinga surfactant.

WO03/106376 (Norsk Hydro, 2003) discloses the use of an additive,preferably a C₆-C₃₀ straight chain fatty acid, such as myristic acid,being temperature stable and amphoteric, to obtain a homogeneous mixedphase.

WO2014/009326 (Shell, 2014) discloses mixing a first flow comprising aliquid fertilizer with a second flow comprising liquid elemental sulphurin a mixing device in the presence of a multifunctional ionic surfactantto form an emulsion comprising elemental sulphur particles which arecoated with a layer of the surfactant and dispersed in a fertilizermaterial that can be solidified.

ReSulf® is an example of a commercial product, sold by YaraInternational ASA, being a particulate urea-based fertilizer comprisingsmall phases of elemental sulphur with a 42-9S composition, and producedfrom a micro-emulsified elemental sulphur in a liquid urea basis using asurfactant and solidified using a classical prilling technique.

Not only prills, but also pastilles of a urea-based fertilizercomprising elemental sulphur were produced by Yara International ASA(Oslo, Norway) with a 42-9S composition using a cooling belt (Sandvik,Stockholm, Sweden and in Nitrogen+Syngas 313, September-October 2011).

It would be advantageous to have an elemental sulphur-containingparticulate urea-based fertilizer which not only has a high nutrientcontent and a rich N:S ratio from an agronomical point of view, but alsocontains elemental sulphur in a form and with a particle size that ismore readily and quickly available as a plant nutrient. Such afertilizer can be applied and can be effective early in the growingseason or at other times.

All of the known methods to solve the aforementioned problem focus onthe use of a homogenous mixed melt and/or the use of a surfactant tominimize elemental sulphur phase size.

STATEMENT OF THE INVENTION

Surprisingly, the inventors now found a method that obviates both theuse of an additive, in particular a surfactant, that improves thehomogeneity of the melt of molten urea-based base material and moltenelemental sulphur, and/or decreases the average particle size of theelemental sulphur phase therein (compared to a melt of molten urea-basedbase material and molten elemental sulphur that does not comprise suchadditive), and the use of a homogenously mixed melt, but still producesparticles, in which the elemental sulphur phases have an average size ofsmaller than about 100 μm, which is about the maximum size over whichthe elemental sulphur becomes too slowly available for the plants. Theuse of a surfactant complicates the prior art procedures and addscompounds to the fertilizer that are not desired on the field and haveno agricultural value. The use of a mixing device increases residencetime in the system (pies, mixing device, pumps, etc.), which should bekept minimal, i.e. a few seconds rather than minutes, in order tominimize decomposition of a urea melt, in particular into biuret andammonia according to the reaction 2 CO(NH₂)₂=>biuret+NH₃.

The method according to the invention is based on the use of a ureafluidized-bed granulator in which liquid urea-based material andelemental sulphur are mixed, sprayed through one or more spraying meanscomprising at least one nozzle and solidified into particles.

An advantage of the method according to the invention is that it can beimplemented in a common urea plant that uses the aforementionedfluidized bed granulation technology without substantial processmodifications or the integration of further equipment, such as a meltmixer, and without the need for the addition of additives, such assurfactants, in particular ionic surfactants, that improve thehomogeneity of the melt of molten urea-based base material and moltenelemental sulphur, and/or decrease the particle size of the elementalsulphur phase therein.

In its broadest concept, the invention is concerned with a method forthe manufacture of a homogeneous, solid, particulate, urea-basedmaterial comprising elemental sulphur, the method comprising the stepsof:

-   -   (i) providing a melt of molten urea-based material and molten        elemental sulphur; and    -   (ii) spraying the melt in a urea fluidized bed granulator using        spraying means such that the melt is solidified into a        homogeneous, solid, particulate urea-based material comprising        solid elemental sulphur phases therein.

According to one embodiment, the invention is concerned with a methodfor the manufacture of a homogeneous, solid, particulate, urea-basedmaterial comprising elemental sulphur, the method comprising theconsecutive steps of:

(a) providing a first liquid flow comprising a urea-based base materialat a first temperature at least at or above the melting temperature ofthe urea-based base material;

(b) providing a second liquid flow comprising elemental sulphur at asecond temperature at least at or above the melting temperature of theelemental sulphur;

(c) continuously joining the first flow with the second flow at a thirdtemperature, at which both flows are liquid, such that elemental sulphurin the resulting melt is in liquid form;

(d) spraying the resulting melt in a urea fluidized bed granulator usingspraying means such that the melt is solidified into a homogeneous,solid, particulate urea-based material comprising solid elementalsulphur phases therein.

In a particular embodiment, the method as described herein is providedwith the proviso that no homogeneity improving additives and/or particlesize decreasing additives are added in the method. As mentioned herein“homogeneity improving additives” refer to additives such as surfactantsand in particular ionic surfactants, that improve the homogeneity of themelt of molten urea-based base material and molten elemental sulphur. Asmentioned herein “particle size decreasing additives” refer to additivessuch as surfactants and in particular ionic surfactants, that decreasethe particle size of the elemental sulphur phase therein.

In a further embodiment, the invention is also concerned with ahomogeneous, solid, particulate urea-based material comprising elementalsulphur comprising elemental sulphur phases in a urea-based basematerial and formed by an accretion process, wherein said elementalsulphur phases have very small sizes in the order of 10 μm or less, andpreferably manufactured according to the method according to theinvention.

In a particular embodiment, the homogeneous, solid, particulateurea-based material as described herein is provided with the provisothat said material comprises no homogeneity improving additives and/orparticle size decreasing additives.

Said particulate urea-based material may advantageously be used as afertilizer, in particular for stimulating the growth of agriculturalproducts on a sulphur-deficient soil.

Said particulate urea-based material may advantageously also be used asan animal feed.

Within the context of this invention, although in the literature, thenature of the elemental sulphur phase is always referred to as particleor droplet, in the context of this invention, the elemental sulphurphase is mostly referred to as a phase that may have a number of shapes,being irregular, droplet-like, flake-like, needle-like, etc.

The invention is not limited to urea-based fertilizers but can also beused for alternate products where elemental sulphur would bebeneficially added to urea. For example, the homogeneous, solid,particulate urea-based material of the present invention could be usedas an animal feed.

In a further embodiment, the invention is also concerned with the use ofa fluidized-bed granulator, equipped with spraying means, for theproduction of a urea-based material which comprises small elementalsulphur phases, preferably wherein elemental said elemental sulphurphases have a size, determined by laser diffraction analysis andexpressed as d90 of smaller than about 20 μm, or expressed as d50 ofsmaller than about 10 μm, or expressed as d10 of smaller than about 5μm.

DETAILED DESCRIPTION OF THE INVENTION

The method will now be described in more detail.

In the late 1970's, fluidized-bed granulation technology has beenintroduced into the field of urea production as an alternative toclassical methods, such as prilling and drum-granulation. A fluidizedbed is formed when a quantity of a solid particulate substance (usuallypresent in a holding vessel) is placed under appropriate conditions tocause a solid/fluid mixture to behave as a fluid. This is usuallyachieved by the introduction of pressurized fluid through theparticulate medium. This results in the medium having many propertiesand characteristics of normal fluids, such as the ability to free-flowunder gravity, or to be pumped using fluid type technologies. Theresulting phenomenon is called fluidization. Fluidized beds are used forseveral purposes, such as fluidized bed reactors (types of chemicalreactors), fluid catalytic cracking, fluidized bed combustion, heat ormass transfer or interface modification, such as applying a coating ontosolid items, and fluidized bed granulation.

There are many types of fluidized-bed granulation, but for the purposesherein, it is only necessary to discuss in detail fluidized-bedgranulation technology which is used to produce urea-based material, inparticular urea. Currently, there are essentially three main differentprocesses used: a process that was originally developed by YaraFertilizer Technology (YFT, Norway) but is now licensed by UhdeFertilizer Technology (UFT, The Netherlands), a process by ToyoEngineering Corporation (TEC, Japan), and a process by Stamicarbon (TheNetherlands). Furthermore, there are some emerging processes, e.g. byUreaCasale (Switzerland) and Green Granulation Technology (GGT, TheNetherlands).

All processes function in essentially the same way, in that—withreference to this invention—individual particles of solid urea,optionally comprising elemental sulphur, are maintained in an agitatedstate by a flow of air, grow by repeatedly being impacted by droplets ofa solution or melt of urea, optionally comprising elemental sulphur,which then solidifies on their surface through a combination ofevaporation (if water is present) and cooling by the fluidizing airbefore the particles come into contact with more urea melt, optionallycomprising elemental sulphur. This is achieved by ensuring the rightturbulence and mixing so that the granules cycle alternatively through azone where the conditions are favourable to the impactation of droplets,and a zone where the conditions are favourable to the solidificationthereof. They are designed to do this without any internal moving parts.The granulation mechanism (accretion process) in fluidized-bed ureagranulation processes differ from that of conventional fertilizergranulation processes (agglomeration process), in which small solidparticles become stuck together by a liquid phase which then solidifiesand cements the small particles together into larger granules. Using thefluidized-bed granulation technology, urea granules, optionallycomprising elemental sulphur, thus comprise a number of layers made ofaccreted solidified droplets, similar to the skins of an onion. Allprocesses are designed to achieve this, but they achieve this in adifferent way. In the UFT process, the urea particles, optionallycomprising elemental sulphur, are maintained in a state of fluidizationby a bulk flow of air, blowing through a perforated plate in the bottomof the granulator box. At regular intervals, through the bed, the ureamelt, optionally comprising elemental sulphur, is atomised in a seriesof spraying means, comprising nozzles surrounded by atomisation air,discharging vertically upwards. These jets serve the dual purpose ofboth spraying droplets of liquid urea on the particles and promotingcirculation in the fluidized bed, such that the particles are sucked inand entrained in the air flow, where they acquire layers of urea melt,optionally comprising elemental sulphur, then pass into a part of thebed where they only encounter fluidized air, which serves to dry, cooland solidify their most recently acquired layer before they areentrained by the next jet along. The TEC technology uses a spouting orebullating bed. It does not use atomisation nozzles as such, instead,the urea melt, optionally comprising elemental sulphur, issuing fromjets in the bottom of the bed is broken up by high-velocity secondaryair, introduced around them. This also lifts entrained particles abovethe top surface of the surrounding fluidized bed. The Stamicarbonprocess does not use atomizing nozzles either; the particles acquiretheir coating of urea, optionally comprising elemental sulphur, by adifferent mechanism, in which they pass through a film of urea melt,optionally comprising elemental sulphur, created by an annular nozzlearound a secondary air jet. All three processes produce granules betweenabout 2 and 8 mm (for more information: see “Fair wind for FBTechnology,” pp. 40-47, Nitrogen+Syngas 282, July-August 2006). TheUreaCasale technology is based on a vortex-type granulator, which isbasically a floating fluidized bed, wherein the particles are fluidizedwith air fed, from the bottom through a grid, such that the particleshave a longitudinal and a circular motion. The urea melt, optionallycomprising elemental sulphur, is sprayed from the side into the rotatingfluidized bed by special spraying means, comprising nozzles, wherein asmall amount of air is injected into the nozzle, such that an emulsionof air in the melt is formed.

Notwithstanding to the above mentioned processes, the invention is notlimited to these processes, but comprises all processes where particlesare substantially formed by accretion. Essential to the invention isthat the formation of the particles is done substantially by the actionof accretion, not agglomeration. The inventors have now found that thistechnology can be used to manufacture a homogeneous, solid, particulate,urea-based material comprising elemental sulphur phases which have anaverage size of smaller than about 100 μm, in particular smaller thanabout 20 μm, in particular smaller than about 10 μm, in particularsmaller than about 5 μm. Without being bound by theory, it is believed(and will be shown later) that the accretion mechanism generates fineelemental sulphur phases within the urea-based particle with an averagesize of about 100 μm or less, preferably with a size of smaller than 50μm or less, more preferably with a size of smaller than 25 μm or less,most preferably with a size of smaller than 10 μm or less and even morepreferably with a size of smaller than 5 μm, such that an elementalsulphur phase of this small size can be readily oxidized to providenutrient sulphur for the plants when applied to the soil.

In its broadest concept, there is provided a method for the manufactureof a homogeneous, solid, particulate, urea-based material comprisingelemental sulphur, the method comprising the steps of:

-   -   (i) providing a melt of molten urea-based base material and        elemental sulphur; and    -   (ii) spraying the melt in a urea fluidized bed granulator using        spraying means such that the melt is solidified into        homogeneous, solid, particulate urea-based base material        comprising solid elemental sulphur phases therein.

The above method can be implemented as a batch process or as acontinuous process. In the batch process, the mixture of moltenurea-based base material and elemental sulphur is provided at atemperature in the range of about 120° C. to 150° C. The mixture may beadvantageously be composed by adding elemental sulphur, for example inpowder form, to a melt of base material, optionally in the presence of asmall amount of water, such as about 5 to 10 weight % or less, andoptionally in the presence of additives, such as anti-caking additives,surfactants, colorants, minor- and trace nutrients, anti-degradationadditives, urease-inhibitors, etc.

According to another embodiment, there is provided a method for themanufacture of a homogeneous, solid, particulate, urea-sulphur materialcomprising elemental sulphur, the method comprising the consecutivesteps of:

-   -   (a) providing a first liquid flow comprising a urea-based base        material at a first temperature at least at or above the melting        temperature of the urea-based base material;    -   (b) providing a second liquid flow comprising elemental sulphur        at a second temperature at least at or above the melting        temperature of the elemental sulphur;    -   (c) continuously joining the first flow with the second flow to        form a third flow at a third temperature at which both flows are        liquid, such that elemental sulphur in the resulting melt is in        liquid form;    -   (d) spraying the resulting melt in a urea fluidized bed        granulator using spraying means such that the melt is solidified        into homogeneous, solid, particulate urea-based material        comprising solid elemental sulphur phases therein.

According to another embodiment, there is provided a method for theproduction of a homogeneous, solid, particulate, urea-based materialcomprising elemental sulphur which comprises the steps of providing afirst liquid flow comprising a urea-based base material at a firsttemperature in the range of about 120° C. to 145° C., providing a secondliquid flow comprising elemental sulphur at a second temperature in therange of about 120° C. to 150° C., joining the first flow with thesecond flow at a third temperature in the range of about 120° C. to 150°C., and spraying the resulting melt in a urea fluidized bed granulatorusing spraying means such that the melt is solidified into ahomogeneous, solid, particulate urea-based material comprising solidelemental sulphur phases therein, for example at a temperature of 95° C.to 120° C.

Molten urea-based base material and molten elemental sulphur areobtained from a source of molten urea-based base material and a sourceof molten elemental sulphur, respectively. Within the context of thisinvention, molten urea-based base material also comprises an aqueoussolution comprising a high concentration of a urea-based base material,such as having a water content of 0.2 to 10 weight %, preferably 3 to 5weight %. The molten urea-based base material is maintained at atemperature that depends on its water content. Typically, thetemperature is about 130° C. and preferably at a temperature in therange of about 120° C. to 145° C. The molten elemental sulphur is alsomaintained at a temperature above its melting point, usually at atemperature above about 120° C. The molten elemental sulphur ismaintained preferably at a temperature in the range of about 120° C. to150° C. An amount of molten urea-based base material and an amount ofmolten elemental sulphur from their respective sources are combined inthe proportions required to yield the desired grade of material.

The flow rate and pressure of the flow of molten urea-based basematerial and those of the flow of molten elemental sulphur can becontrolled separately and in the relation to each other, as will becomeapparent, such that the desired quantity of material can be produced,the amount of elemental sulphur sufficient is to obtain the desiredgrade of material and the elemental sulphur phases in the urea-basedmaterial have the desired size. For example, the flows of elementalsulphur/urea-based base material can be joined in a flow ratio whichranges between 0.1:100 and 25:100 by weight, preferably between 1:100and 15:100 by weight, such that particles are formed that contain from0.1 to 20 weight % of elemental sulphur, preferably from 1 to 10 weight% of elemental sulphur.

The grades of the homogeneous, solid, particulate, urea-based materialcomprising elemental sulphur may vary over a broad range. Grades with aslittle as a few weight % of elemental sulphur, for example a grade with4 weight % of elemental sulphur (a 43-4S composition) or with as much asabout 20 weight % of elemental sulphur (a 37-20S composition) can bemanufactured. For most agricultural applications, the N:S weight ratioranges between 4:1 to 10:1, corresponding to about 5 to 10 weight % ofelemental sulphur compared to the total weight of the particles in thecase of a urea/sulphur fertilizer.

Since the two flows are physically insoluble with each other, theresulting flow is only a mechanically mixed and inhomogeneous combinedflow. The joining of the respective flows may be accomplished by any oneof a number of methods. For example, the appropriate amounts of moltenelemental sulphur and molten urea-based base material may be supplied tothe suction of a suitable pump. The combined amounts of molten elementalsulphur and molten urea-based base material are then passed by the pumpdirectly to the granulator, in particular to the spraying means whichcomprise at least one nozzle. There is no need for a separate mixingdevice to obtain a homogenized melt of small molten elemental sulphurphases in molten urea-based base material, as long as the joining of themolten urea-based base material flow and the molten elemental sulphurflow is continuous, i.e. not interrupted for a period long enough toproduce a plug of either molten urea-based base material or moltenelemental sulphur to produce particles that comprise predominantly ofeither urea-based base material or elemental sulphur.

Hence, according to one embodiment, a method according to the inventionis described wherein the melt of molten urea-based base material andmolten elemental sulphur is inhomogeneous. One way of measuring(in)homogeneity at a certain location in the process is by measuring thesulphur content (weight %) or the U/S ratio (weight % of urea/weight %of elemental sulphur) in a number of samples, e.g. sampled from thethird flow, and determining the sum of squares of deviations of all datapoints from their sample mean (DEVSQ) for said S content or U/S ratio(e.g. as defined as the DEVSQ function in Microsoft Excel). Preferably,the DEVSQ of said S content is more than 1, preferably more than 5, inparticular between 1 and 30 (determined on at least 5 samples of about 2gram). Preferably, the DEVSQ of said U/S ratio is more than 1,preferably more than 3, in particular between 1 and 15 (determined on atleast 5 samples of about 2 gram).

The method according to the invention produces a homogeneous, solid,particulate, urea-based material comprising elemental sulphur. Thehomogeneity of said material can be determined in the same way asdescribed above. Preferably, the DEVSQ of the S content of said materialis less than 1, preferably less than 0.5 (determined on at least 5samples of about 2 gram).

In an alternative embodiment, the respective flows may be provided to amixing device, such as a vessel, provided with agitation, anhomogenizer, a static mixer, a mixing pump or a “T”-shaped device asdescribed in U.S. Pat. No. 4,330,319, wherein the combined flows aremixed before the resulting melt is being transferred to the granulatordevice, in particular the spraying means comprising at least one nozzle.

In order to maintain the desired temperatures and to prevent anypremature solidification of molten material, all apparatus that containmolten elemental sulphur and/or urea-based base material can be, forexample, steam-traced, internally or externally, or steam-jacketedand/or insulated.

The combined urea-based base material/sulphur melt is subsequentlysolidified into solid particulates of homogeneous urea-based basematerial comprising elemental sulphur, wherein the elemental sulphurphase has an average size of smaller than about 100 μm, in particularsmaller than about 20 μm, in particular smaller than about 10 μm, inparticular smaller than about 5 μm, by passing the combined urea-basedbase material/sulphur melt to the spraying means comprising at least onenozzle of the urea fluidized bed granulator. The nozzle can be anynozzle that is appropriate for the respective granulator. For example,good results were obtained with a UFT granulator equipped with BETEspiral-type atomization nozzles (BETE Fog Nozzle, Inc., Greenfield, USA)and/or with HFT-type atomization nozzles (EP 1701798 B1, 2005, YaraInternational ASA) at an operating pressure of about 0.5 bar and a flowrate of about 10 litres/min. Note that such nozzle is operated at a muchlower pressure than the nozzles, disclosed in U.S. Pat. No. 4,330,319(Cominco Ltd, 1982) which nozzles need a pressure drop of at least about200 kPa (2 bar). The use of a lower pressure is an advantage as lessenergy is required for spraying the melt.

It is desirable to maintain the residence time of the melt prior tospraying as short as possible. Thus, the time that elapses between thejoining step and the melt leaving the spraying means in the fluidizedbed granulator should preferably be as short as possible. For example, aresidence time of the melt between the steps of joining and spraying inthe order of about 10 to 100 seconds or less will ensure thatdegradation of urea to biuret is minimal.

The solidified particles are subsequently screened and a product ofdesired particle sizes is recovered. The product is a homogeneous,solid, particulate urea-based material comprising elemental sulphurcomprising a uniform dispersion of small elemental sulphur phases in aurea-based base material, wherein the elemental sulphur phases have anaverage size of smaller than about 100 μm, in particular smaller thanabout 20 μm, in particular smaller than about 10 μm, in particularsmaller than about 5 μm.

To reduce the tendency of caking of the solid urea-based materialcomprising elemental sulphur, a suitable anti-caking agent may be used,such as formaldehyde. A small amount of a suitable agent may be appliedto the solidified particles or to the product size fraction as desired,by coating or spraying. Alternatively and preferably, a suitableanti-caking agent may be added either to the source of molten urea-basedbase material, the molten mixture of urea-based base material andelemental sulphur, to the flow of molten urea-based base material priorto either the urea pump or the joining of the urea-based base materialwith the elemental sulphur flow. Typically formaldehyde or ureaformaldehyde is added to the urea-based base material melt prior togranulation to serve this purpose and act as a granulating agent.

Optionally, further additives may be added, such as colorants, minor-and trace nutrients, anti-degradation additives, urease-inhibitors, etc.

According to one embodiment, the method according to the inventionprovides a urea-based material, wherein the urea-based base material isselected from the group of urea, urea-ammonium sulphate, andurea-ammonium phosphate fertilizer.

The preferred embodiments of the method according to the invention willnow be illustrated by means of the following non-limitative examples.

EXAMPLES

All experiments were performed on the urea pilot plant in Sluiskil (YaraInternational ASA). This pilot plant has a batch capacity—aftersieving—of about 50 kg of on-spec product. It basically consists of astirred urea preparation vessel with an active volume of about 150litres and a fluidized bed granulator of the UFT-type, equipped with aspraying nozzle of the spiral-type or HFT-type (disclosed in EP 1701798B1, 2005, Yara International ASA).

Analytics

-   -   Melt concentration was measured via Karl Fischer analysis on a        flake sample taken from the prepared melt in the mixing vessel.    -   S-content was measured with a LECO type SC 144 DR analyser        (LECO, Saint Joseph, Mass., USA) and by weight after filtration.    -   pH was determined via titration analysis.    -   The d50 of the granules was determined via sieving or with a        Retsch Camsizer Particle Analyzer (Retsch Technology GmbH, Haan,        Germany).    -   PQR caking index was measured with a pneumatic caking machine        with 2 bar pressure for 24 hours at 27° C.    -   PQR crushing index was measured with a small scale up to 10 kg        capacity and granules were crushed on the scale with a flat        ended steel rod.    -   PQR abrasion dust was measured with a dust formation apparatus,        containing a glass column, two air inlet valves, a glass head, a        flow-meter and a gauze with mesh app. 1 mm.    -   Apparent density was measured with a GeoPyc 1360 pycnometer of        Micromeritics (Norcross, Ga., USA).    -   The particle size distribution of the elemental sulphur phases        in the urea granules was measured via laser diffraction analysis        and secondly via wind sieve mill analysis. Laser diffraction        analysis was done using a Cilas 1180 instrument with wavelengths        of 635 and 835 nm (Cilas, Orleans, France). Analysis of the        diffraction data was carried out using the Mie-theory, according        to ASTM method D4464-10 “Standard Test Method for Particle Size        Distribution of Catalytic Material by Laser Light Scattering”        with a complex refractive index of 1.9+i0.01.    -   Sample preparation for laser diffraction analysis: 80 g of        granules comprising urea and elemental sulphur phases (also        called particles) were dissolved in 500 ml of deionized water at        circa 60° C. for 2 hours with stirring. The resulting suspension        comprising the solid elemental sulphur particles was filtered        and washed with warm deionized water. The thus extracted solid        elemental sulphur particles were dispersed in isopropanol        (volume of 15 mL) and treated with ultrasonic agitation (probe        of 750 W, 20 kHz) for 20 minutes before the laser diffraction        analysis was performed. The ultrasonic agitation disagglomerates        those solid elemental sulphur particles that are agglomerated in        the solution. Without this treatment, slightly higher values for        the particle size are obtained.    -   The total surface area of the extracted solid elemental sulphur        particles as a dry elemental sulphur powder was carried out on a        Micromeritics 3Flex volumetric adsorption system using the BET        method. Krypton was adsorbed at 77K, according to ASTM method        D4780—12 “Standard Test Method for Determination of Low Surface        Area of Catalysts and Catalyst Carriers by Multipoint Krypton        Adsorption”.    -   Assuming either spherical or cubic particle shape, the mean        particle size from the surface area was calculated by:

Size (m)==6/[Density (g·m⁻³)·Area (m²·g⁻¹)]

-   -   where Size is the diameter of a spherical particle or the edge        length of a cubic particle. The density of elemental sulphur is        2.0×10⁶ g·m⁻³.

Sample preparation for total surface area determination: 80 g ofgranules comprising urea and elemental sulphur particles were dissolvedin 500 ml of deionized water at circa 60° C., for 2 hours with stirring.The resulting solution comprising the solid elemental sulphur particleswas filtered and washed with warm deionized water. The extracted solidelemental sulphur particles were dried over-night at 80° C.

Example 1: Urea+5 Weight % S (Spiral Nozzle)

122.14 kg of liquid urea was shortly mixed (not enough to obtain ahomogeneous mixture) in a vessel containing a stirrer, with 6.5 kg ofelemental sulphur in powder form and 1.36 kg of a urea-formaldehydeconditioning agent, UF80 (from Dynea AS, Lillestrøm, Norway), which is amixture of urea/formaldehyde/water, in a ratio of 23/57/20) at atemperature of about 129° C. to obtain a melt mix with 5% of elementalsulphur with a melt concentration of 96.2%, after which the resultingmelt was pumped to an active fluidized bed granulator at a granulationtemperature of about 104° C., equipped with a spiral nozzle. Theinjection time was about 14 minutes. The particulate urea-basedfertilizer comprising elemental sulphur was discharged from thegranulator, sieved and cooled to room temperature. A representativesample of the product was analysed to determine the characteristics ofthe particles. The results are given in Table I and II.

Example 2: Urea+10 Weight % S (Spiral Nozzle)

115.71 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 13.0 kg of elementalsulphur in powder form and 1.29 kg of a urea-formaldehyde conditioningagent, UF80 (from Dynea AS, Lillestrøm, Norway), which is a mixture ofurea/formaldehyde/water in a ratio of 23/57/20) at a temperature ofabout 130° C. to obtain a melt mix with 10% of elemental sulphur with amelt concentration of 95.2%, after which the resulting mixture waspumped to an active fluidized bed granulator at a granulationtemperature of about 101° C., equipped with a spiral nozzle. Theinjection time was about 13 minutes. The particulate urea-basedfertilizer comprising elemental sulphur was discharged from thegranulator, sieved and cooled to room temperature. A representativesample of the product was analysed to determine the characteristics ofthe particles. The results are given in Table I and II.

Example 3: Urea+5 Weight % S (HFT Nozzle)

122.14 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 6.5 kg of elementalsulphur in powder form and 1.36 kg of a urea-formaldehyde conditioningagent, UF80 (from Dynea AS, Lillestrøm, Norway), which is a mixture ofurea/formaldehyde/water in a ratio of 23/57/20) at a temperature ofabout 130° C. to obtain a melt mix with 5% of elemental sulphur with amelt concentration of 96.3%, after which the resulting mixture waspumped to an active fluidized bed granulator at a granulationtemperature of about 108° C., equipped with an HFT nozzle. The injectiontime was about 13 minutes. The particulate urea-based fertilizercomprising elemental sulphur was discharged from the granulator, sievedand cooled to room temperature. A representative sample of the productwas analysed to determine the characteristics of the particles. Theresults are given in Table I and II.

Example 4: Urea+10 Weight % S (HFT Nozzle)

115.71 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 13.0 kg of elementalsulphur in powder form and 1.29 kg of a urea-formaldehyde conditioningagent, UF80 (from Dynea AS, Lillestrøm, Norway), which is a mixture ofurea/formaldehyde/water in a ratio of 23/57/20) at a temperature ofabout 130° C. to obtain a melt mix with 10% of elemental sulphur with amelt concentration of 97.4%, after which the resulting mixture waspumped to an active fluidized bed granulator at a granulationtemperature of about 108° C., equipped with an HFT nozzle. The injectiontime was about 13 minutes. The particulate urea-based fertilizercomprising elemental sulphur was discharged from the granulator, sievedand cooled to room temperature. A representative sample of the productwas analysed to determine the characteristics of the particles. Theresults are given in Table I and II.

Example 5: Urea+11 Weight % S (Spiral Nozzle, Injection Air Flow 230kg/h)

115.93 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 14.3 kg of elementalsulphur in pastille (3-6 mm) form and 1.28 kg of a urea-formaldehydeconditioning agent, UF80 (from Dynea AS, Lillestrøm, Norway), which is amixture of urea/formaldehyde/water in a ratio of 23/57/20) at atemperature of about 130° C. to obtain a melt mix with 11% of elementalsulphur with a melt concentration of 95.1%, after which the resultingmixture was pumped to an active fluidized bed granulator at agranulation temperature of about 107° C., equipped with a spiral nozzle.The injection time was about 12 minutes. The particulate urea-basedfertilizer comprising elemental sulphur was discharged from thegranulator, sieved and cooled to room temperature. A representativesample of the product was analysed to determine the characteristics ofthe particles. The results are given in Table I and II.

Example 6: Urea+11 Weight % S (Spiral Nozzle, Injection Air Flow 170kg/h)

115.93 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 14.3 kg of elementalsulphur in pastille (3-6 mm) form and 1.28 kg of a urea-formaldehydeconditioning agent, UF80 (from Dynea, which is a mixture ofurea/formaldehyde/water in a ratio of 23/57/20) at a temperature ofabout 130° C. to obtain a melt mix with 11% of elemental sulphur with amelt concentration of 95.6%, after which the resulting mixture waspumped to an active fluidized bed granulator at a granulationtemperature of about 105° C., equipped with a spiral nozzle. Theinjection time was about 11 minutes. The particulate urea-basedfertilizer comprising elemental sulphur was discharged from thegranulator, sieved and cooled to room temperature. A representativesample of the product was analysed to determine the characteristics ofthe particles. The results are given in Table I and II.

Example 7: Urea+5 Weight % S+10 Weight % AS (Spiral Nozzle, InjectionAir Flow 170 kg/h)

110.5 kg of liquid urea was mixed (not enough to obtain a homogeneousmixture) in a vessel containing a stirrer, with 13.0 kg of crystallineammonium sulphate and 6.5 kg elemental sulphur in pastille (3-6 mm) formand 1.21 kg of a urea-formaldehyde conditioning agent, UF80 (from DyneaAS, Lillestrøm, Norway), which is a mixture of urea/formaldehyde/waterin a ratio of 23/57/20) at a temperature of about 130° C. to obtain amelt mix with 5 weight % of elemental sulphur and 10 weight % ammoniumsulphate with a melt concentration of 95.7%, after which the resultingmixture was pumped to an active fluidized bed granulator at agranulation temperature of about 105° C., equipped with a spiral nozzle.The injection time was about 12 minutes. The particulate urea-ammoniumsulphate fertilizer comprising elemental sulphur was discharged from thegranulator, sieved and cooled to room temperature. A representativesample of the product was analysed to determine the characteristics ofthe particles. The results are given in Table I and II and compared to atypical urea sample, obtained in the same way

TABLE I Quality analysis Ex. 7 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 5%S + 5% S 10% S 5% S 10% S 11% S 11% S 10% AS Urea Nozzle type SpiralSpiral HFT HFT Spiral Spiral Spiral Injection air flow (kg/h) 230 230   230 230    230 170 230 Moisture (%) 0.17 0.20 0.12 0.12 0.18 0.21 0.170.15-0.25 S-content (%) 4.9 8.6 ^(a) 4.5 8.5 ^(a) 10.8 10.4 5.0 0   pH7.8 7.9  7.5 7.5  7.6 7.9 3.8 8.5-9.7 PQR caking index (kgf, at 27° C.)20 29    32 38    — — 65 20-65 PQR crushing index (kg) 4.2 4.3  4.4 4.1 4.2 3.8 3.9 4.0-4.6 PQR Impact resistance (%) 0.9 0.7  0.7 0.6  1.7 0.90.9 0.4-5  PQR abrasion dust formation (mg/kg) 400 1025     2250 4100^(b)    400 900 2100 100-200 d50 granules (mm) (mg/kg) 3.31 3.39 3.093.22 3.29 3.31 3.29 3.39 Apparent density (g/cm³) 1.29 1.29 1.28 1.251.30 1.29 1.22 1.25 ^(a) some loss occurred ^(b) possibly due to highermelt concentration and lower moisture level.

TABLE II S-particle size Ex. 7 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 5%S + 5% S 10% S 5% S 10% S 11% S 11% S 10% AS Typical Nozzle type SpiralSpiral HFT HFT Spiral Spiral Spiral Injection air flow (kg/h) 230 230230 230 230 170 170 Laser diffraction analysis d10 (μm) 3.06 2.67 2.663.29 3.76 2.70 3.80 about 5 or lower d50 (μm) 7.08 6.54 6.53 7.67 8.577.23 9.07 about 10 or lower d90 (μm) 13.85 13.79 13.10 14.95 17.61 16.6718.18 about 20 or lower surface-weighted mean D[3.2]^(C) (μm) 19.7 19.220.4 ≈20 — — — about 20 or lower volume-weighted mean D[4.3]^(C) (μm)28.5 30.3 41.2 ≈40 — — — about 40 or lower Krypton BET Surface area(m²/g) 0.617 0.471 0.633 0.405 0.419 0.419 0.339 Calculated averageparticle size (μm) 4.7 6.4 4.6 7.4 7.1 6.9 8.6 about 10 or lower Windsieve mill analysis <32 μm 86% — — — — — — about 90% <20 μm 70% — — — —— — about 70% ^(C)these experiments were done without ultrasonictreatment to disagglomerate the particles

From these experiments, it can be concluded that the method according tothe invention delivers a high quality product, of which thecharacteristics are comparable to the ones of urea. Selection of theprocessing conditions should reduce dust formation during production.

Surprisingly, it was found that more than 70% of the S-phases was <20μm. In particular, the size (d50) of the S-phases was less than 10 μm.This is considerably smaller than the size of the S-particles disclosedin U.S. Pat. No. 4,330,319, where only 5.7% and 7.5% of the S-particleshad a particle size diameter of about 20 μm. Via laser diffractionanalysis, for all produced urea-based fertilizer comprising elementalsulphur, a d50 was found between 6.5 and 8.2 μm. This matched almostperfectly with the calculated average particle size from the BET surfacearea, i.e. 5.0 to 7.5 μm. Hence, the method according to the inventiondoes not only provides a more efficient process, it also providesparticles with smaller S-phases than the prior art particles. Asmentioned, a small elemental sulphur phase size is favourable for anefficient bacterial conversion into sulphates.

Example 8

For all samples, the moisture uptake was measured over time usingmonolayer analysis at 20° C./80% relative humidity for 24 hours. Theresults, shown in FIG. 1, show a similar moisture uptake behaviour asregular urea granules.

Example 9

To determine the (in)homogeneity of the smelt and the granules,determined as the DEVSQ (the sum of squares of deviations of data pointsfrom their sample mean) of the S-content or Urea/Sulfur ratio,urea/elemental S samples (flakes, about 2 gram) were taken from the meltpreparation in the mixing vessel and after circulation, close to theinjector in the granulator, for the mixture of Example 5 (11 weight %sulphur) and from the resulting granules (about 2 gram of granules persample). As can clearly be seen, the melt present in the mixing vesselis very inhomogeneous (high DEVSQ). Once circulated at higher flows withmore turbulence, the melt becomes more homogeneous (lower DEVSQ). Thefinal granules are homogeneous (DEVSQ less than 1).

TABLE 3 Homogeneity Sample (about 2 g) Elemental Sulphur (weight %) U/SMixing vessel 1 13.9 6.89 2 15.2 5.59 3 9.1 10.03 4 10.7 8.33 5 14.55.87 Mean 12.7 7.3 Std 2.64 1.8 DEVSQ 27.9 13.6 Injector 1 13.6 6.36 211.3 7.82 3 10.9 8.20 4 11.2 7.95 5 10.2 8.77 Mean 11.4 7.8 Std 1.28 0.9DEVSQ 6.6 3.2 Granules 1 10.4 — 2 10.6 — 3 10.8 — 4 10.4 — 5 10.4 — 610.4 — 7 10.6 — 8 10.5 — 9 10.7 — 10 10.7 — 11 10.3 — 12 10.3 — 13 10.6— 14 10.3 — Mean 10.5 — Std 0.17 — DEVSQ 0.36 —

It will be understood that modifications can be made to the embodimentsof the invention described and illustrated therein without departingfrom the scope of the invention as defined in the appended claims.

Furthermore, also the following aspects of the invention are disclosed:

Aspect 1.

-   -   A method for the manufacture of a homogeneous, solid,        particulate, urea-based material comprising elemental sulphur        phases, the method comprising the steps of:        -   (i) providing a melt of molten urea-based base material and            molten elemental sulphur; and        -   (ii) spraying the melt in a urea fluidized bed granulator            using spraying means such that the melt is solidified into a            homogeneous, solid, particulate urea-based material.

Aspect 2.

-   -   A method for the manufacture of a homogeneous, solid,        particulate, urea-sulphur material comprising elemental sulphur        phases, the method comprising the consecutive steps of:        -   (a) providing a first liquid flow comprising a urea-based            base material at a first temperature at least at or above            the melting temperature of the urea-based base material;        -   (b) providing a second liquid flow comprising elemental            sulphur at a second temperature at least at or above the            melting temperature of the elemental sulphur;        -   (c) continuously joining the first flow with the second flow            to form a third flow at a third temperature at which both            flows are liquid, such that elemental sulphur in the            resulting melt is in liquid form;        -   (d) spraying the resulting melt in a urea fluidized bed            granulator using spraying means such that the melt is            solidified into a homogeneous, solid, particulate urea-based            material.

Aspect 3.

The method according to any one of aspect 1 to 2, wherein thesolidification of the particulate urea-based material is done in thegranulator substantially by the action of accretion.

Aspect 4.

The method according to any one of aspects 2 to 3, wherein the firsttemperature is in the range of about 120° C. to 145° C., and/or whereinthe second temperature is in the range of about 120° C. to 150° C.,and/or wherein the third temperature is in the range of about 120° C. to150° C.

Aspect 5.

The method according to any one of aspects 1 to 4, wherein the step ofspraying the resulting melt in a urea fluidized bed granulator usingspraying means such that the melt is solidified into a homogeneous,solid, particulate urea-based material in performed at a temperature of95° C. to 120° C.

Aspect 6.

The method according to any one of aspects 2 to 5, wherein the ratio ofsecond flow to first flow ranges between 0.1:100 and 25:100 by weight,preferably between 1:100 and 15:100 by weight.

Aspect 7.

The method according to any one of aspects 2 to 6, wherein the residencetime of the third flow between the steps of joining and spraying is inthe order of about 10 to 100 seconds.

Aspect 8.

The method according to any one of aspects 1 to 7, wherein the sprayingmeans comprise at least one atomization nozzle, operated at less than 2bar, preferable less than 1 bar.

Aspect 9.

The method according to any one of aspects 1 to 8, wherein the sulphurphases in the homogeneous, solid, particulate urea-based material havesizes, expressed as surface-weighed mean D[3.2] of smaller than about 40μm, preferably smaller than about 30 μm, more preferably smaller thanabout 20 μm.

Aspect 10.

The method according to any one of aspects 1 to 8, wherein the sulphurphases in the homogeneous, solid, particulate urea-based material sizes,expressed as volume-weighed mean D[4.3] of smaller than about 50 μm,preferably smaller than about 40 μm, more preferably smaller than about30 μm.

Aspect 11.

The method according to any one of aspects 1 to 10, wherein theurea-based base material is selected from the group of urea,urea-ammonium sulphate, and urea-ammonium phosphate fertilizer.

Aspect 12.

A homogeneous, solid, particulate urea-based material, obtainable by themethod according to any one of aspects 1 to 11.

Aspect 13.

A homogeneous, solid, particulate urea-based material, comprising finelydivided elemental sulphur phases in a urea-based base material.

Aspect 14.

A homogeneous, solid, particulate urea-based material according toaspect 12 or 13, wherein said sulphur phases have a size, expressed assurface-weighed mean D[3.2] of smaller than about 40 μm, preferablysmaller than about 30 μm, more preferably smaller than about 20 μm.

Aspect 15.

A homogeneous, solid, particulate urea-based material according toaspect 12 or 13, wherein said sulphur phases have a size, expressed asvolume-weighed mean D[4.3] of smaller than about 50 μm, preferablysmaller than about 40 μm, more preferably smaller than about 30 μm.

Aspect 16.

Use of the homogeneous, solid, particulate urea-based material asdisclosed in aspects 12 to 15 as a fertilizer.

Aspect 17.

Use of the homogeneous, solid, particulate urea-based material asdisclosed in aspects 12 to 15 as for supporting the growth ofagricultural products on a sulphur—deficient soil.

Aspect 18.

Use of the homogeneous, solid, particulate urea-based material asdisclosed in aspects 12 to 15 as an animal feed.

1-20. (canceled)
 21. A method for the manufacture of a homogeneous,solid, particulate, urea-based material comprising elemental sulphur,the method comprising the steps of: (i) providing a melt of moltenurea-based base material and molten elemental sulphur; and (ii) sprayingthe melt in a urea fluidized bed granulator using spraying means,wherein the spraying means comprise at least one atomization nozzle,operated at less than 2 bar, such that the melt is solidified intohomogeneous, solid, particulate urea-based material comprising solidelemental sulphur phases therein; wherein the solid elemental sulphurphases have a size, determined by laser diffraction analysis andexpressed as d90, of smaller than about 20 μm, or expressed as d50, ofsmaller than about 10 μm, or expressed as d10, of smaller than about 5μm; wherein the solidification of the particulate urea-based material isdone in the fluidized bed granulator substantially by the action ofaccretion.
 22. The method according to claim 21, wherein step (i)comprises: (a) providing a first liquid flow comprising a urea-basedbase material at a first temperature at least at or above the meltingtemperature of the urea-based base material; (b) providing a secondliquid flow comprising elemental sulphur at a second temperature atleast at or above the melting temperature of the elemental sulphur; and(c) continuously joining the first flow with the second flow to form athird flow at a third temperature at which both flows are liquid, suchthat elemental sulphur in the resulting melt of molten urea-based basematerial and molten elemental sulphur is in liquid form.
 23. The methodaccording to claim 21, wherein the melt of molten urea-based basematerial and molten elemental sulphur is sprayed in the absence of anadditive that improves the homogeneity of the melt of molten urea-basedbase material and molten elemental sulphur, and/or decreases the averageparticle size of the elemental sulphur phases therein.
 24. The methodaccording to claim 21, wherein the melt of molten urea-based basematerial and molten elemental sulphur is inhomogeneous, wherein theDEVSQ of the S content is more than 1 (determined on at least 5 samplesof about 2 gram).
 25. The method according to claim 22, wherein thefirst temperature is in the range of about 120° C. to 145° C., and/orwherein the second temperature is in the range of about 120° C. to 150°C., and/or wherein the third temperature is in the range of about 120°C. to 150° C.
 26. The method according to claim 21, wherein the step ofspraying the resulting melt in a urea fluidized bed granulator usingspraying means such that the melt is solidified into a homogeneous,solid, particulate urea-based material is performed at a temperature of95° C. to 120° C.
 27. The method according to claim 22, wherein theratio of second flow to first flow ranges between 0.1:100 and 25:100 byweight.
 28. The method according to claim 22, wherein the residence timeof the third flow between the steps of joining (c) and spraying (ii) isin the order of about 10 to 100 seconds.
 29. The method according toclaim 21, wherein about 90% of the elemental sulphur phases have a size,determined by wind sieve mill analysis, of smaller than 32 μm.
 30. Themethod according to claim 21, wherein the urea-based base material isselected from the group consisting of urea, urea-ammonium sulphate, andurea-ammonium phosphate fertilizer.
 31. The method according to claim21, wherein the homogeneity, determined as the DEVSQ for the S contentof said material is less than 1 (determined on at least 5 samples ofabout 2 gram).
 32. The method according to claim 21, wherein thespraying means comprise at least one atomization nozzle, operated atless than 1 bar.
 33. A homogeneous, solid, particulate urea-basedmaterial, comprising finely divided elemental sulphur phases in aurea-based base material and formed by an accretion process wherein saidelemental sulphur phases have a size, determined by laser diffractionanalysis and expressed as d90, of smaller than about 20 μm, or expressedas d50, of smaller than about 10 μm, or expressed as d10, of smallerthan about 5 μm; and wherein the homogeneous, solid, particulateurea-based material does not comprise a surfactant.
 34. A homogeneous,solid, particulate urea-based material according to claim 33, whereinabout 90% of the elemental sulphur phases have a size, determined bywind sieve mill analysis, of smaller than 32 μm.
 35. The homogeneous,solid, particulate urea-based material according to claim 33, whereinthe homogeneity, determined as the DEVSQ for the S content of saidmaterial is less than 1 (determined on at least 5 samples of about 2gram).
 36. A fertilizer, for supporting the growth of agriculturalproducts on a sulphur-deficient soil, or as an animal feed, comprisingthe homogeneous, solid, particulate urea-based material as claimed inclaim 33.